This invention relates to a process for manufacturing high precision electrical resistance elements from metallic foils or thin films adhered to an insulating substrate by using an ion beam for etching predetermined amounts of foil from the substrate. The invention also relates to the electrical resistance elements produced according to the manufacturing process of the invention.
There are at least several well-known etching processes for manufacturing electrical resistance elements from metallic layers adhered to insulating substrates. In a typical process, a masking material is applied on the exposed surface of the metallic layer to produce a mask in a masking pattern corresponding to the shape of the connected metallic filaments to be produced by etching away the unmasked metallic layer material. After etching and cleaning, only the connected metallic filaments having a small cross-section remain. By an appropriate choice of a masking pattern, it is possible to obtain relatively long connected filament lengths within a relatively small surface area of the substrate. Correspondingly, a resistance element thus produced has a high ohmic value per unit of substrate surface area. The etching of the unmasked metallic layer is accomplished by chemical or electrochemical etching steps as disclosed, for example, in U.S. Pat. No. 3,174,920, or by an ion beam etching step. The latter process is described, for example, in an article entitled "Masking for Ion Beam Etching" by P. Gloersen, Solid State Technology (April, 1976) pp. 68-73.
There are a number of disadvantages in the prior art etching processes which are overcome by the present invention. The prior art etching processes employ the mask and masking material as a shield to inhibit or prevent etching of the masked portion of the metallic layer material in the basic etching step. Such processes thus require an additional process step, usually involving a chemical bath, to remove the masking material after the basic etching step has been completed, as described, for example, in the Gloersen article. An additional problem of the prior chemical, electrochemical, and ion beam etching processes is that such processes usually require that careful dimensional changes be made to the basic mask and masking pattern in order to change the electrical properties of the resistive elements being produced.
The chemical and electrochemical etching processes, in addition, produce relatively irregular edges on the metallic filaments. Such irregularities adversely affect the electrical properties of the filaments, and make it difficult for those processes to be used to produce large numbers of resistive elements having electrical properties which are uniform within precision tolerances.
A broad objective of the present invention is to overcome these and other disadvantages of the prior art etching processes. In particular, the present invention allows the manufacture of resistive elements with a high ohmic value per unit of substrate surface area in a process having at least the following advantages over the prior art. First, the electrical properties of the resistive elements being manufactured are reproduceable within precision tolerances in large volume manufacture. Second, a separate process step for the removal of the masking material is eliminated. Third, the electrical properties of the resistive elements being manufactured may be customized during manufacture by simple process control without any need to make dimensional changes in the basic mask or masking pattern. These advantages are realized by deviating from the prior art teaching that the purpose of the masking material is to act as a shield to prevent etching of the metal by the basic etching process.